Chef Cookbook Programming Tutorials – notifies and subscribes explained with examples

A notification is a property on a resource that listens to other resources in the resource collection and then takes actions based on the notification type (notifies or subscribes).

A timer specifies the point during the chef-client run at which a notification is run. The following timers are available:

Specifies that the action on a notified resource should be run before processing the resource block in which the notification is located.

Default. Specifies that a notification should be queued up, and then executed at the very end of the chef-client run.

Specifies that a notification should be run immediately, per resource notified.

A resource may notify another resource to take action when its state changes. Specify a ‘resource[name]’, the :action that resource should take, and then the :timer for that action. A resource may notify more than one resource; use a notifies statement for each resource to be notified.

The syntax for notifies is:

notifies :action, ‘resource[name]’, :timer

The following examples show how to use the notifies notification in a recipe.

template ‘/etc/nagios3/configures-nagios.conf’ do

  # other parameters

  notifies :run, ‘execute[test-nagios-config]’, :delayed

end

By default, notifications are :delayed, that is they are queued up as they are triggered, and then executed at the very end of a chef-client run. To run an action immediately, use :immediately:

template ‘/etc/nagios3/configures-nagios.conf’ do

  # other parameters

  notifies :run, ‘execute[test-nagios-config]’, :immediately

end

and then the chef-client would immediately run the following:

execute ‘test-nagios-config’ do

  command ‘nagios3 –verify-config’

  action :nothing

end

A resource may listen to another resource, and then take action if the state of the resource being listened to changes. Specify a ‘resource[name]’, the :action to be taken, and then the :timer for that action.

Note that subscribes does not apply the specified action to the resource that it listens to – for example:

file ‘/etc/nginx/ssl/example.crt’ do

   mode ‘0600’

   owner ‘root’

end

service ‘nginx’ do

   subscribes :reload, ‘file[/etc/nginx/ssl/example.crt]’, :immediately

end

In this case the subscribes property reloads the nginx service whenever its certificate file, located under /etc/nginx/ssl/example.crt, is updated. subscribes does not make any changes to the certificate file itself, it merely listens for a change to the file, and executes the :reload action for its resource (in this example nginx) when a change is detected.

The syntax for subscribes is:

subscribes :action, ‘resource[name]’, :timer

The following examples show how to use the subscribes notification in a recipe.

Prevent restart and reconfigure if configuration is broken

Use the :nothing action (common to all resources) to prevent the test from starting automatically, and then use the subscribes notification to run a configuration test when a change to the template is detected:

execute ‘test-nagios-config’ do

  command ‘nagios3 –verify-config’

  action :nothing

  subscribes :run, ‘template[/etc/nagios3/configures-nagios.conf]’, :immediately

end